1. Field of the Invention
The present invention generally relates to a hub for a local area network (hereinafter referred to as LAN), more specifically relates to a backup construction of a power supply for a hub for maintaining its function even when power failure due to such an accident as a defect in power supply wiring occurs.
2. Background Art
A typical LAN employs a transmission line of 10BASE standards. Among them, a transmission line according to 10BASE-T standard has been widely implemented these days since it is easier to wire and more flexible in moving or increasing a node in the network such as a personal computer (hereinafter referred to as PC). In a 10BASE-T LAN, an unshield twisted pair wire (hereinafter referred to as UTP), which is similar to an usual telephone line wire, is used as a transmission line. Connection of the UTP is easier than those in other 10BASE standards. Each of the wires extended from LAN boards installed in the nodes such as PCs is simply connected to a communication port of a hub. The hub is typically defined as a common wiring point for star-topology network and has a similar functions to those of a line concentrator.
The basic functions of a hub includes electrical isolation of the nodes such as PCs connected to the communication ports using a signal processing circuit, transmission of the signals communicated among the nodes, and reshaping a waveform of those signals. Such high performance hubs as an intelligent hub or a switching hub provide additional functions as a multi-layer connection using a plurality of hubs and connection with a backbone.
The above-mentioned hubs usually operate with a commercial ac power supply. The supplied ac current is converted into a stabilized dc current by a power circuit such as an ac/dc converter provided in the hub to drive the load circuit including a signal processing circuit. An ordinary hub is not equipped with a power switch, therefore, the hub becomes operative when connected to an ac power supply. As a result, once connected to an ac power supply, the hub is continuously powered thereby. In general, the hub does not require frequent maintenance and access after installation. Thus, the hub is sometimes disposed below a floor panel with the wiring.
A computer system responsible for an important information processing usually employs a backup power supply system so that abrupt power failure does not affect the system. In a small-scale system, an uninterruptible power supply, hereinafter referred to as UPS, is usually provided with each of the equipments in a network such as PCs and hubs. The UPS is an apparatus connected to a commercial ac power supply so as to provide an ac current inverted from an output dc current of a built-in backup battery.
However, there are some shortcomings with employment of the UPS. One is that the UPS is large in size. Another is that the UPS is expensive. Due to these problems, the following disadvantages arise: significant increase in cost of a power supply facility; difficulty in determining appropriate arrangement of the UPS in a system and capacity thereof; awkwardness in wiring of the power supply cables; and necessity for a large space for installation of the UPS.
In view of the above problems, the present applicant has reached a basic concept of a hub provided with a built-in a backup function of a power supply. More specifically, like a notebook-sized PC, a backup secondary battery is equipped with the hub, which is trickle charged with a supplied ac current via a charging circuit and provides a dc output current for driving the load circuit when an ac power supply fails. The conventional apparatus such as a notebook-sized PC having a backup secondary battery has a manual power switch. While the manual power switch is opened, operation of the apparatus and discharging of the secondary battery are prevented except a small amount of self discharge of the battery.
However, since the hub does not have a power switch as previously mentioned, a conventional backup power supply circuit using a secondary battery has the following problem when applied to the hub.
The hub without a power switch is in operation continuously by a fully charged backup secondary battery since it is shipped. Therefore, the backup secondary battery installed during an assembling process is exhausted after several days depending on a capacity of the backup battery. Because of this problem, a user of the hub is required to charge the exhausted secondary battery so as to activate a backup power supply function before starting to use the hub, although the backup power supply function of the hub should become effective immediately after it is connected to a network system.